The present invention relates to a laser diode driving method and circuit for controlling an optical output and extinction ratio at a constant level in correspondence with deterioration of the laser diode with time.
An example of a laser diode driving circuit used in an optical transmission system or the like is disclosed in Japanese Patent Laid-Open No. 2-308584 in which the optical output from the laser diode is controlled at a constant level regardless of the ambient temperature.
FIG. 5 shows the arrangement of a conventional laser diode driving circuit for controlling the optical output from the laser diode.
A laser diode LD is driven by a current prepared by superposing a driving current Id and a bias current Is. The driving current Id is a pulse current based on transmission data, whereas the bias current Is is a base current for causing the LD to emit light by induced emission. A temperature sensor 110 generates a voltage corresponding to the ambient temperature and outputs the voltage as an analog temperature signal representing the ambient temperature to an A/D converter 120. The A/D converter 120 converts the input temperature signal into a digital signal and outputs the digital signal to a memory 150.
The memory 150 uses input digital signal as an address signal to read out digital data stored at the corresponding address from the memory 150 and output the data to a D/A converter 130. The D/A converter 130 converts the input digital data into an analog signal and outputs the analog signal to a current controller 140. The current controller 140 controls an emitter current Is common to transistors Q1 and Q2 in accordance with the analog signal from the D/A converter 130.
The operation of the laser diode driving circuit will be described.
A pre-bias signal is applied to the base of the transistor Q1. Assume that the state in which the pre-bias signal voltage is higher than a reference voltage (-VR) is a disable state. In the disable state, the transistor Q1 is turned on, the transistor Q2 is turned off, and the laser diode LD is not driven. Assume that the state in which the pre-bias signal voltage is lower than the reference voltage (-VR) is an enable state. In the enable state, the transistor Q1 is turned off, the transistor Q2 is turned on, and the laser diode LD is driven by a current, i.e., a current which changes between Is and Is+Id, prepared by superposing the driving current Id and the bias current Is generated by the current controller 140.
In the memory 150, data about the bias current corresponding to the ambient temperature is stored. When data obtained by digitally converting a temperature signal representing the ambient temperature is input from the A/D converter 120 to the address line of the memory 150, the memory 150 outputs data about the bias current corresponding to the ambient temperature to the data line.
The D/A converter 130 D/A-converts the bias current data output to the data line, and outputs the analog signal to the current controller 140. The current controller 140 controls the emitter current of the transistors Q1 and Q2 in accordance with the analog signal output from the D/A converter 130.
In the laser diode driving circuit, the emitter current Is is adjusted in correspondence with the ambient temperature. That is, when the ambient temperature changes, data on the address line changes, and data about a new bias current appears on the data line. The D/A converter 130 D/A-converts the data on the data line, and the current controller 140 converts the signal output from the D/A converter 130 into a current.
At this time, if the pre-bias signal changes to the enable state, the laser diode LD is driven by a current prepared by superposing the driving current Id on the new bias current Is adjusted in correspondence with the ambient temperature.
The laser diode driving circuit employs a feed forward controller. Since the circuit performs control for only optical output fluctuation conditions set in advance, it cannot control the optical output from the laser diode in correspondence with optical output fluctuation conditions other than ambient temperature fluctuations. For this reason, e.g., when the laser diode deteriorates with time to decrease the optical output, the optical output may be smaller than its lower limit defined in the optical transmission system.
In the laser diode driving circuit, light emission may delay, the extinction ratio may decrease, and the quality of the transmission system may degrade because no consideration is given to temperature fluctuations in differential quantum efficiency of the laser diode. That is, the optical output is controlled at a constant level by changing only the bias current without changing the driving current in correspondence with the ambient temperature.
FIGS. 1A and 1B show the current vs. optical output characteristics of a general laser diode. FIG. 1A shows current vs. optical output characteristics when the bias current and the driving current are ideally distributed. FIG. 1B shows current vs. optical output characteristics when the driving current is kept constant.
In FIGS. 1A and 1B, t1, t2, and t3 (t1&lt;t2&lt;t3) represent ambient temperatures; Isn, Idn, and Ithn (n=1, 2, 3), the bias current, the driving current, and the light emission threshold current of the laser diode; and Po, the optical output. The laser diode driving circuit controls the optical output Po at a constant level at the respective temperatures. The laser diode has such a characteristic that both the bias current Is and driving current Id required to obtain a constant optical output Po increase along with an increase in ambient temperature. As shown in FIG. 1A, it is ideal for efficiently driving the laser diode that the bias current Is is set about the light emission threshold current Ith of the laser diode, and the driving current Id is superposed on the bias current Is to keep the optical output constant.
In the laser diode driving circuit in FIG. 5 for controlling the optical output at a constant level by changing the bias current Is while keeping the driving current Id constant, if the bias current Is2 and the driving current Id2 are optimum at an ambient temperature t2, but the ambient temperature decreases to t1, the set value Is1 of the bias current becomes smaller than the light emission threshold current Ith1 to delay light emission. If the ambient temperature increases from t2 to t3, the set value Is3 of the bias current exceeds the light emission threshold current Ith3 to decrease the extinction ratio, failing to obtain a reliable extinction state.